All-fiber Mach-Zehnder interferometers typically include two optical couplers separated by a phase shift region, which comprises two optical fibers that interconnect, said couplers. The two fibers, which are often referred to as “arms” have different optical path lengths so that optical signals propagate through them at different velocities in the phase shift region. Light launched into the device passes through the first coupler where it is split and led through the pair of optical fibers. Both lightwaves are then coupled again by the second coupler and taken out as an optical signal output from the two output ports of the second coupler. If the light portions recombining at the second coupler are in phase, they constructively interfere at one of the output ports of the second coupler; if they are not in phase, in particular if the two light portions incur a π differencing phase shift, they combine constructively at the other output port of the second coupler.
Mach-Zehnder interferometers are known for their narrow band capabilities. For example, they can be used in dense wavelength division multiplexer (DWDM) optical communication systems. For this purpose, they must be stable over a range of environmental conditions, such as temperatures, within a defined range, and during presence of temperature variations. However, the refractive indices or the optical path lengths of the two connecting fibers of the device between the two couplers will usually vary with temperature. If the temperature dependence of the indices of refraction of the two fibers is not equal or if the optical paths of the two fibers are not equal, the temperature variations will cause variations in the differential phase shift. Consequently, the channel spacing of the device, defined as the wavelength separation between the transmission peaks of wavelengths of two adjacent channels, as well as the wavelength peaks and passband, become unstable, which causes significant problems for DWDM applications due to the small separation between channels in DWDMs.
In view of the importance of MZ-type interferometer devices, it is highly desirable to have available such devices that can exhibit stable performance even in the presence of some thermal disturbances. This can be achieved by compensating for the temperature induced shift so as to maintain the optical path length difference unchanged as the temperature varies.
Efforts have been made in the past to design Mach-Zehnder interferometers and other fiber optic devices so as to achieve high thermal stability and minimize temperature variations and other thermal effects.
For example, U.S. Pat. No. 4,725,141 provides an all-fiber MZI with connecting fibers or arms between the couplers being of equal length and located close to each other, thus ensuring that the effects of temperature changes are minimized since both arms are equally affected by temperature variations. In such case, however, the connecting arms must be made of the same material and to achieve the required phase shift a transducer is coupled to at least one of the interferometer arms, which is not a very practical feature.
U.S. Pat. No. 6,118,909 discloses a different manner by which optical devices having a plurality of waveguides of differing lengths, such as wavelength routers, may be treated to achieve improved temperature independence. This is done by applying a temperature-compensating material, such as a polymer, on selected areas of the device thereby varying the cross-sections of the waveguides to improve temperature independence. Such procedure is not straightforward, since it is difficult to access the evanescent field, i.e. to apply the polymer near the core of the fiber.
Finally, U.S. Pat. No. 6,031,948 describes a temperature compensation technique of an all-fiber Mach-Zehnder interferometer, where two connecting fibers are of different lengths. This is achieved by mounting the shorter fiber on a composite substrate, such that, as temperature rises, the substrate expands to increase the tension and length of the shorter fiber in order to maintain a constant path length difference, or the longer fiber is mounted on a composite substrate such that, as the temperature rises, the substrate contracts to decrease the tension and length of the longer fiber and thereby preserve the desired path difference. This is essentially a packaging technique, which proves to be complex, since it requires delicate adjustments and mechanical fabrication, for example, when the connecting fibers between the couplers are essentially of the same length.
Thus, there is still a need for an all-fiber MZI with a passive thermal compensation that would allow controlling the thermal dependence of the device within a desired temperature range in a precise and accurate way, without adding complexity to the packaging of the component.